In various systems such as optical fibers for home use and a local area network (LAN), a silicon-based optical communication device operating at an optical fiber communication wavelength of 1330 nm or 1500 nm is a very promising technique capable of enabling an optical functional element and an electronic circuit to be integrated on a silicon platform using a complementary metal oxide semiconductor (CMOS) technique.
In recent years, extensive research has been conducted on silicon-based passive devices such as waveguides, optical couplers, and wavelength filters. Further, as an important technique by which an optical signal for a communication system can be operated, there are silicon-based active devices such as optical modulators and optical switches, and such active devices are receiving much attention. However, optical modulators and optical switches that change a refractive index using a thermo-optical effect of silicon are low in speed, and can be used only at a device speed of up to a modulation frequency of 1 Mbps. Thus, in order to implement a high modulation frequency required in many optical communication systems, an optical modulator or an optical switch using an electro-optical effect of enabling a high-speed operation is required.
In the case of pure silicon, a linear electro-optical effect (Pockels effect) does not occur and a change in the refractive index of pure silicon by a Franz-Keldysh effect or a Kerr effect is very small. For this reason, most electro-optical modulators currently proposed change a real part and an imaginary part of the refractive index and change the phase and intensity of light propagated through a silicon layer by changing the free carrier density of the silicon layer using a carrier plasma effect. In a modulator using such free carrier absorption, the intensity of output light is directly modulated by a change in absorption of light propagated through a silicon layer. Further, as a structure using a change in refractive index, a structure using a Mach-Zehnder interferometer is common. In a Mach-Zehnder interferometer of a waveguide type, an intensity modulation signal of light is obtained by performing propagation through two arms based on a change in a refractive index or giving a phase difference between light beams and causing light beams to interfere with each other.
In an electro-optical modulator, a free carrier density can be changed by injection, accumulation, removal, or inversion of free carriers. However, in most electro-optical modulators already under review, optical modulation efficiency is poor, a length (hereinafter referred to simply as an “optical phase modulation length”) necessary for optical phase modulation is a millimeter (mm) order, and an injection current density higher than 1 kA/cm3 is necessary. In the case of an electro-optical modulator, when an optical phase modulation length is long and an element size is large, it is easily affected by a temperature distribution on a silicon platform, and an original electro-optical effect is likely to be removed due to a change in a refractive index of a silicon layer caused by a thermo-optical effect. Thus, in order to implement miniaturization, high integration, and low power consumption in an electro-optical modulator, an element structure having high optical modulation efficiency is required.
As an electro-optical modulator satisfying the above requirements, for example, a silicon-based electro-optical device including a rib waveguide structure on an SOI substrate is disclosed in Non-Patent Document 1. In the silicon-based electro-optical device disclosed in Non-Patent Document 1, slab regions extending in a traverse direction at both sides of a rib waveguide structure
including an intrinsic semiconductor region are doped to have a p type and an n type, respectively.
The rib waveguide structure is formed using a silicon layer 1S on an SOI substrate including a support substrate 3 made of silicon and an embedding oxide layer 2 as illustrated in FIG. 14. The rib waveguide structure is a PIN diode modulator, and is a structure in which a free carrier density in an intrinsic semiconductor region is changed by applying forward and reverse biases, and a refractive index is changed using a carrier plasma effect. In the PIN diode modulator of FIG. 14, intrinsic semiconductor silicon 1 is formed to include p+ doped semiconductor silicon 4 formed by performing a doping process on the silicon layer 1S coming in contact with a first electrode contact layer 6A at a high concentration. Further, the intrinsic semiconductor silicon 1 includes n+ doped semiconductor silicon 5 formed by performing a doping process on the silicon layer 1S at a high concentration and a second electrode contact layer 6B coming in contact with the n+ doped semiconductor silicon 5.
The p+ and n+ doped semiconductor silicon 4 and 5 is subjected to a doping process showing a carrier density of about 1020 per cubic centimeter (cm3).
In the rib waveguide structure illustrated in FIG. 14, the first and second electrode contact layers 6A and 6B are connected to a power source (not shown) via an electrode interconnection 7. As a forward bias is applied to the PIN diode using the first and second electrode contact layers 6A and 6B, free carriers are injected into the waveguide. Then, as free carriers increase, the refractive index of the intrinsic semiconductor silicon 1 changes, and thus phase modulation of light propagated in the waveguide is performed.
Further, as another electro-optical modulator, for example, a silicon-based, electro-optical device including a silicon-insulator-silicon (SIS) structure formed on an SOI platform is disclosed in Patent Document 1. The silicon-based electro-optical device disclosed in Patent Document 1 includes n doped polycrystalline silicon 10 serving as a main body region formed on a relatively thin silicon surface layer of an SOI substrate and p doped semiconductor silicon 9 serving as a gate region stacked to partially overlap the n doped polycrystalline silicon 10 as illustrated, in FIG. 15. Further, a relatively thin dielectric layer 12 is formed on a stacked interface between the p doped semiconductor silicon 9 and the n doped poly crystalline silicon 10. The p and n doped polycrystalline silicon 9 and 10 that is subjected to the doping process in the gate region and the main body region is specified such that a change in a carrier density is controlled by an external signal voltage applied via an electrode interconnection 7 and p+ and n+ doped semiconductor silicon 4 and 11.